Complete conversion of lactate to methane and CO2 by triculture with a sulfate reducer, strain F-l, a hydrogen-utilizing methanogen, strain HM-l, and an acetate-utilizing methanogen, strain AM-l, was studied with sulfate-free medium under N2 and CO2. Strain F-1 was motile, Gram-negative and vibrio-shaped cells which utilized H2, lactate and ethanol as electron donors. Strain F-1 were hybridized with the oligodeoxynucleotide probe complementary to specific regions of mesophilic Gram-negative

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... ate-reducing bacteria (SRB probe). The G+C content of the DNA was 63.2mol%. Strain F-1 was tentatively classified into Desulfovibrio vulgaris. Strain F-1 was unable to degrade lactate in the sulfate-free medium under N2 and CO2. However, it degraded lactate in the dual culture with strain HM-1 in sulfate-free medium. Lactate was completely converted to methane and carbon dioxide by triculture with strain F-l, strain HM-l and strain AM-1 which was a filamentous acetateutilizing methanogen. The triculture formed flocs and grew well on the bottom of culture flasks. Acetate accumulated in the dual culture was used concurrently with its production in the triculture. Acetate conversion to methane by strain AM-1 was not inhibited by hydrogen. Lactate was converted to methane and CO2 stoichiometrically by the triculture in a sulfate-free minimal medium. The 16S rRNA-targeted oligodeoxynucleotide fluorescent probe in situ hybridization method was used for identification and enumeration of strain F-1 in the microbial triculture ecosystem. A large number of strain F-1 cells hybridized with the SRB probe were observed while lactate was consumed at the early stage of incubation.